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AMC1350: Input offset more than max rating. Design error?

Md RASOOL
Prodigy 20 points
Part Number: AMC1350
Other Parts Discussed in Thread: AMC1300

Tool/software:

Greetings,

I am using AMC 1350 and AMC1300 for sensing current and voltages and I am reading more than max rated offset errors in multiple instances. One of AMC1350 has an offset of 10mV DC at the inputs when resistor divider is shorted. Reading voltage across a resistor of 1M ohm with 10M ohm in series. The error is generally over the max rated value when tested on multiple AMC1350s. I have to short the differential input pins, bypassing resistor network to make it 0 and read 0mV on the other side.

The VCC is maintained at approximately 5.3V on both sides, with ripples under 2mV. I am measuring AC and DC voltages around 10V and trying to not burden the circuit I am sensing and hence upped my resistor bridge from 2.27MOhm to 11MOhm. U9 is my AMC1350 in the pic below.

My issues are same as this post: AMC3311: Offset problems due to interference and bad layout? - Amplifiers forum - Amplifiers - TI E2E support forums 

I would greatly appreciate any insights or suggestions you might have to address these discrepancies. Your expertise is invaluable to us, and any guidance on mitigating these offset errors would be highly beneficial.

  • 0
    TI__Expert 6415 points

    Hi Rasool,

    Thank you for your questions. Would you be able to provide a schematic of your system? I'd like to see the filter design. The antialiasing filter should have a cutoff frequency of 2MHz or under. This information could help me understand where the discrepancy may be stemming from.

    Are you seeing the offset error on just the AMC1350s or also the AMC1300s? One initial thought I have regarding offset errors is to ensure either your INP or INN has a DC current path to GND1 to define the common-mode voltage. 

    Please let me know.

    Best regards,

    Eva

  • 0 in reply to Eva Mason
    Prodigy 20 points

    Thanks for the quick response, Eva. Please check the schematics below. The issue is with the AMC1350. I have the same filters for (50/60Hz) AC and DC measurements for AMC1350 and do a software filtering on ADC outputs. The filter is on the higher side for faster response to fluctuations.

    Best regards,

    M G Rasool

  • 0 in reply to Md RASOOL
    TI__Expert 6415 points

    Hi M G Rasool,

    I think the cutoff frequency may be too low, what is your input? An antialiasing frequency between 200kHz and 2MHz is recommended. Typically, we see something closer to 10nF for the antialiasing capacitor. 

    Please let me know if you can try this.

    Best regards,

    Eva

  • 0 in reply to Eva Mason
    Prodigy 20 points

    Will try this and update you as soon as I get back.

  • 0 in reply to Md RASOOL
    TI__Expert 6415 points

    Great, thank you!

    Best regards,

    Eva